1. Institute of Materials and Surface Engineering, Faculty of Materials Science and Applied Chemistry, Riga Technical University, P. Valdena Street 3, Riga LV1048, Latvia
2. Department of Physical Chemistry, University of Latvia, LV1004 Jelgavas Street 1, Riga, Latvia
3. Institute of Solid State Physics, University of Latvia, Kengaraga Street 8, Riga LV1063, Latvia